Various movement or position encoders for sensing linear, rotary or angular movement are currently available. These encoders are generally based on optical systems, magnetic scales, inductive transducers, or capacitive transducers.
Optical encoders may utilize a self-imaging arrangement in order to detect a displacement of a scale member comprising a scale grating. The basic principle of self-images, also known as Talbot images, is described in the paper by Cowley, J. M., and A. F. Moodie, 1957, Proc. Phys. Soc. B, 70, 486, which is incorporated herein by reference. An exemplary optical encoder utilizing self-imaging is disclosed in U.S. Pat. No. 6,906,315 (the '315 patent), which is incorporated herein by reference in its entirety.
Optical encoders may utilize an interferometric arrangement in order to detect a displacement of a scale member comprising a scale grating. For optical encoders utilizing an interferometric arrangement, a number of systems have been developed. One recent system utilizing fewer parts than most previous systems is disclosed in U.S. Pat. No. 5,909,283, to Eselun, which is incorporated herein by reference in its entirety. The system described in the '283 patent has a grating scale and readhead including a point source (laser diode in readhead), a Ronchi grating or holographic element which acts as a spatial filter, and a photodetector array. As described, the point source results in interference fringes having a fringe pitch equal to that of the scale. The interference fringe light is transmitted through the Ronchi grating or holographic element to the photodetector array. The photodetector array is arranged to derive four channels of quadrature signals from the transmitted fringe light. In another example, U.S. Pat. No. 7,126,696 (the '696 patent), which is incorporated herein by reference in its entirety, discloses an optical encoder using an interferometric arrangement.
However, despite the foregoing, there is an ongoing need for higher resolution optical encoders which are economical, robust, and easy to install. One method of providing higher resolution is to use a scale member comprising a scale grating which provides a very fine scale grating pitch. However, prior art encoders which use this approach have various undesirable limitations. Optical encoders which improved resolution and/or fewer design and assembly constraints would be desirable.